So, I churned to TPG on 1st December, the actual connection was swapped sometime around noon.
Westnet charged me a full December usage. And you need to notify them to close the account (you’d think the fact you can’t connect to it would be sufficient…) Not that they’d answered any of my other e-mails anyway.
I guess the lesson is, make sure your cutover day is the last day of the month? Or does that just mean the new ISP charges you a full month for starting before the 1st of the next month?